Unbreak VC++ build.

llvm-svn: 32113
diff --git a/llvm/lib/ExecutionEngine/Interpreter/ExternalFunctions.cpp b/llvm/lib/ExecutionEngine/Interpreter/ExternalFunctions.cpp
index 1b8f399..161873c 100644
--- a/llvm/lib/ExecutionEngine/Interpreter/ExternalFunctions.cpp
+++ b/llvm/lib/ExecutionEngine/Interpreter/ExternalFunctions.cpp
@@ -27,6 +27,7 @@
 #include "llvm/Target/TargetData.h"
 #include <csignal>
 #include <map>
+#include <cmath>
 using std::vector;
 
 using namespace llvm;